Agro terrorism, the deliberate contamination of the food supply, has been recognized as a major bioterrorism threat that could cause severe economic dislocation. The deliberate release of animal diseases might in many cases also impact human health. Capable of infecting both livestock and humans and causing severe disease, the emerging, highly pathogenic Nipah virus constitutes an agro terrorism threat of high priority. Nipah and measles virus, closely related members of the paramyxovirus family of enveloped RNA viruses, are airborne pathogens that infect cells through merging of the viral envelope with the target cell plasma membrane. This requires a number of interdependent conformational changes in the viral envelope glycoproteins. We propose to explore our structural insights into determinants for paramyxovirus fusion glycoprotein activity towards better understanding the mechanism of Nipah virus infection and the rational development of novel Nipah virus entry inhibitors. We have generated structural models of the related Nipah and measles virus fusion proteins and, through characterizing different measles virus isolates, have identified a micro domain at the neck domain of the fusion protein that is essential for virus entry. Key sequence features of this area are conserved among all paramyxoviruses analyzed and the structural model of the Nipah virus fusion protein shows a similar domain to be present, supporting the prediction that it has a conserved function in paramyxovirus entry and hence constitutes a promising target site for novel antivirals. Conceptual support for this hypothesis comes from the identification of a small molecule compound that was designed to fill the F cavity domain and shows inhibitory activity against live measles and Nipah virus glycoprotein mediated membrane fusion. Towards further exploring the feasibility of our approach, identifying structural determinants for Nipah virus entry, and ultimately developing applicable therapeutics against Nipah virus, we will assess the role of the Nipah virus F cavity in cell entry (aim 1) and characterize the molecular target site of the first generation entry inhibitor (aim 2). Through integration of our experimental data and in silico compound design we will then rationally develop second generation inhibitors with increased specificity against Nipah virus. These will be characterized for their activity against live Nipah (aim 3).